Amplifier circuits



Oct. 8 1940. s ERT AMPLIFIER CIRCUITS Filed April 22. 1938 l T0 REPRODUCER RE AMPLIFIER T0 SIGNAL SOURCE INVENTOR kg 2/ GER T A-T1.'ORNEY L UDW/ Patented Oct. 8, 1940 UNITED STATES 2,217,297 AMPLIFIER CIRCUITS Ludwig Siegert, Berlin-Siemensstadt, Germany,

assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic mit beschrankter Haftung,

Berlin, Germany,

a corporation of Germany Application April 22, 1938, Serial No. 203,569

' In Germany April 27, 1937 I 6 Claims.

It is known that when certain receiver sets are adjusted to an output sound-volume above the average, fluctuations or bubbling happens which consists of a flicker in the acoustic volume occurring at a frequency of around 5 per second. In order to avoid this drawback, an attempt has been made to connect relatively large blocking condensers in parallel relation to the plate potential. It is also known in the art to render the 1 audio portion impermeable for the low relaxation frequency by choosing suitable dimensions for the coupling means. However, these steps frequently prove inadequate or even harmful because they involve the risk of the frequency band being trimmed.

A simpler and safer method to eliminate bubbling of volume arising as a result of the receiver or amplifier tube being run on a high load is here suggested. The invention essentially resides in 20 the fact that the cathode series resistance, or part thereof, belonging to the tube worked on a high load, more particularly the end or power-stage tube, is used to form part of the voltage divider for the screen-grid potential of the input tube.

The operation of the invention shall be explained more fully by reference to the drawing showing an illustrative embodiment of the basic idea. If the power tube L is driven very far, that is, if it is operated on a large load, as the plate current rises to a high value, a stronger fall of potential happens across the high inner resistance of the source of current supply, especially in the case of mains supply, so that the plate potential decreases. Since in the input tubes the screengrid potential is governing for the gain, and since the same forms part of the total plate potential, reaction upon the tubes will be produced. The result is that the entire apparatus is rendered unstable.

In practicing the present invention, the cathode resistance a forms part of a voltage divider a, b, c, for the screen-grid potential of the input tube H. In this exemplified embodiment, only one radio frequency amplifier stage H with associated band- :5 pass filter B is shown, followed by the signal rectifier D and the audio amplifier stage N.

A circuit organization of the kind here disclosed has the following effect: A more powerful current through the power tube L will flow also through- 0 the cathode resistance a, with the result that the potential at point Q in respect of A also increases. This rise of potential occasions also a corresponding rise of potential at point P, that is, of the screen-grid potential, and this equalizes the 5 decrease in the screen-grid potential owing to the drop of the entire plate potential, if the ;re

sistances'a, b, 0, have been suitably chosen. 'It will not be necessary in all instancesto choose the size correctly seeing that a reduction of, the fluctuation of the screen-grid voltage alone may lead to the desired result;

The invention is particularly valuable infre ceiver sets in which amplification is'driven beyond the average limit. For example, it has been ascertained that the shortcoming and the difficulty above mentioned could be suppressed in a receiver with high audio amplification. Where push-pull schemes are used, the risk of a relaxation wave arising is less imminent seeing that the plate A. C. of such a stage does not flow through the source of current supply.

I claim:

1. In a radio receiver of 'the'type comprising a plurality of cascaded signal transmission tubes having a common direct current potential supply source and the first of the tubes being a radio frequency signal transmission tube and including at least a screen grid in addition to a cathode, signal grid and output electrode, the last of said tubes being an audio amplifier, a resistive path connected between a point of positive potential of said source and a point of the source which is negative relative to said positive point, a first portion of said path being in the space current path of the last of said tubes, and a direct current voltage connection from the screen grid of said first tube to an intermediate point on the remaining portion of said resistive path.

2. In a radio receiver of the type including at least one radio frequency screen grid amplifier tube, a detector, an audio amplifier tube, and a common direct current source for the electrodes of said tubes, means for preventing fluctuations of the internal resistance of the audio tube from affecting the stability of the radio amplifier tube, said means comprising a voltage divider resistor connected across the terminals of said current source, a section of the resistor being arranged in the space current path of the audio tube and providing the normal negative grid bias for the latter, and a direct current voltage connection between the screen grid of said radio amplifier tube and a point on the remaining section of said resistor which is positive with respect to said first section.

3. In a radio receiver of the type including at least one radio frequency screen grid amplifier tube, a detector, an audio amplifier tube, and a common direct current source for the electrodes of said tubes, means for preventing, fluctuations in the space current path section, said first section of the resistor being connected between the audio amplifier cathode and the negative terminal of said current source.

4. In a radio receiver of the type comprising a plurality of cascaded signal transmission tubes having a common direct current potential supply source and the first of the tubes being a radio frequency signal transmission tube and including at least a screen grid in addition to the cathode, signal grid and output electrode, the last of said tubes be ng an audio amplifier, a resistive path connected between a point of positive potential of said source and a point of the source which is negative relative to said positive point, a portion of said path being in the space current path of the last of said tubes, and a direct current voltage connection from the screen grid of said first tube to an intermediate point on the remaining portion of said resistive path, said first resistive path portion being connected between the cathode of said last tube and said negative point of the potential supply source,

5. In a modulated signal carrier'receiving system having a'signalcarrier amplifier, a detector and an audio amplifier, saidcarrier amplifier including a tube provided with a screen grid, a common direct current supply source for the tubes of the system, means for preventing instability due to fluctuations in the internal resistance of the audio amplifier tube, said means comprising a bridge network comprising three resistor arms and a fourth arm consisting of said current source, the junction of said fourth arm and an adjacent arm being grounded, the cathode. of said carrier amplifier tube being connected to said grounded junction, the amplifier screen grid being connected to the conjugate point of said bridge,

and the audio amplifier tube having its cathode to plate path connected between the opposite pair of conjugate points of said bridge. 7

6. In a modulated signal carrier receiving syse tem having a signal carrier amplifier, a detector and an audio amplifier, said carrier amplifier including a tube provided with a screen grid, a

common direct current supply source for the tubes of the system, means for preventing instability due to fluctuations in the internal resistance of the audio amplifier tube, said means comprising a bridge network comprising three resistor arms and a fourth arm consisting of said current source, the junction of said fourth arm and an adjacent arm being grounded,the cathode of said carrier amplifier tube being connected to said grounded junction, the amplifier screen grid being connected to the conjugate point of said bridge, and the audio amplifier tube having its cathode to plate path connected between the opposite pair of conjugate points'of said bridge, the signal grid of the audio amplifier tube being connected to the said grounded junction, and the screen grid of said carrier amplifier being connected to the positive terminal of said current source. Y

LUDWIG SIEGERT. 

